1. Field of the Invention
The present invention relates to a machine tool.
2. Description of the Related Art
There exist conventionally know machine tools which are designed to machine a workpiece by moving a cutting tool relative to the workpiece while rotating the cutting tool.
As an example, Japanese Unexamined Patent Publication No. 10-263959 describes a machine tool including a table which is made movable along an X-axis in a horizontal plane with a workpiece placed on the table, a main spindle unit mounted above the table for rotating a cutting tool attached to the main spindle unit, a cross rail supporting the main spindle unit movably along a Y-axis at right angles to a feed direction (X-axis) of the table in a horizontal plane, and a pair of columns installed on opposite sides of the table, the pair of columns supporting the cross rail in such a way that the cross rail can move up and down. The machine tool cuts the workpiece into a desired shape by means of the cutting tool while controllably moving the table, the main spindle unit and the cross rail in different directions.
The machine tool of the aforementioned patent Publication is configured such that the two columns each have a generally C-shaped horizontal cross section with open sides of the columns directed face to face with each other. The two columns support the cross rail, embracing opposite ends thereof within inward-directed openings of the columns. The cross rail supports the main spindle unit in balance from both sides along the X-axis so that the main spindle unit would not lean by its own weight in one direction along the X-axis. This serves to prevent a reduction in machining accuracy due to tilting of the cutting tool attached to the main spindle unit.
One problem of a prior art arrangement shown in the aforementioned patent Publication is that the machine tool requires a large-sized pair of vertical columns. Since these columns each have a generally C-shaped horizontal cross section and support the cross rail by embracing the opposite ends thereof within the inward-directed openings as stated above, each of the columns must have a width larger than that of the cross rail along the X-axis. Additionally, to sufficiently avoid deformation and changes in posture of the cross rail caused by a force exerted thereupon as a result of movement of the main spindle unit in the aforementioned cross rail supporting mechanism, it is necessary for the columns to have increased stiffness to stably support the cross rail. This would result in a further increase in size of the columns.